1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic type such as a printer or a copier.
2. Description of the Related Art
In image forming apparatuses of an electrophotographic type that form images by charging toner and using an electrostatic force, in a case where the amount of charged toner changes, the density or the quality of an output image changes according to the change. The amount of charged toner markedly changes according to the usage environment, the amount of consumption of toner, and an output elapse time. Accordingly, in a case where some kind of control operation for stabilizing the output is not performed, an output image varies depending on a variety of conditions.
In the case of the image forming apparatuses of the electrophotographic type, toner of almost the same amount as the amount of consumption of toner, which is predicted based on image data, is supplied. In addition, in the case of image forming apparatuses of the electrophotographic type using a two-component developing device that forms an image by using toner and a magnetic carrier as developer, generally, the toner supply amount is adjusted or limited using an output value of an inductance sensor that measures the density of toner in the developer.
In the two-component developing device, the amount of charged toner changes according to a mixing ratio of toner to a magnetic carrier contained inside the developing device. Generally, the lower the ratio of toner is, the larger the amount of the charged toner becomes. As the amount of charged toner increases, the amount of toner adhering to an electrostatic latent image having a constant amount of electric charge decreases. On the other hand, as the amount of charged toner decreases, the amount of toner adhering to an electrostatic latent image having a constant amount of electric charge increases. Accordingly, there is a problem in that the amount of charged toner and the density of an output image change based on the consumption and the supply of the toner.
In order to solve such a problem, as disclosed in Japanese Patent Laid-Open No. 09-127757, a patch image used for measuring the density of an output image is output to stabilize the density of the image. A technique has been proposed in which the toner supply amount is controlled to form the amount of charged toner, at which the density of the output image matches a target density, based on the density of the patch image that is measured on an image bearing member. According to such a technique, a mechanism is included which develops the patch image used for a control process and performs control such that a detection result of the density of the patch image is fed back to the toner supply amount.
Then, for a latent image of a predetermined patch image electric-potential, by controlling the toner supply amount such that the density of the patch image is constant, a T/D ratio that is the ratio between toner and the magnetic carrier contained in the two-component developer is changed. As a result, by performing control such that a density reference value of the patch image used for controlling the supply of toner and a read-out density value of the formed patch image match each other, the amount of the charged toner reaches a desired target amount of the charged toner. In other words, the amount of charged toner and the image density can be maintained to be constant through the feedback control of the supply of toner based on the formation of a patch image in a predetermined electric potential condition. However, the amount of charged toner is maintained to be constant only in a case where the developing efficiency does not change.
However, in a case where the toner supply amount is controlled based on the output result of the patch image as above, there may be a case where the action of forming the amount of charged toner to be a target value that is constant is interfered. In the electrophotography technique, since an image is formed by using an electrostatic force, the amount of charged toner can be unchanged. This problem will be described in detail as below.
In a control mechanism for stabilizing the density of the output image through adjusting the toner supply amount based on the output result of a patch image, the density of the patch image is measured to control various adjustments. Accordingly, when the filling ratio (hereinafter, referred to as a “developing efficiency”) of charged toner for the development contrast changes according to the toner temperature, the density of the patch image changes even in a case where the amount of charged toner is the target value.
In other words, the characteristics of the toner surface change according to the toner temperature, and the toner parting properties between the toner and the magnetic carrier changes, whereby the developing efficiency changes so as to change the amount of development (the amount of loaded toner). Even in such a situation, the amount of supply of the toner is controlled such that the density of the patch image matches the target density, and accordingly, the amount of the charged toner deviates from the target value.
This will be described below in detail. As the density of the patch image, the amount of toner included in the patch image is determined based on the amount of reflected light. The amount of toner is proportional to the development contrast (an electric potential difference between the electric potential of an image portion and a developing bias (a DC bias applied to a developing sleeve)) and a developing efficiency (the filling ratio of charged toner for the development contrast) and is inversely proportional to the specific charge of toner (electric charge per unit mass of toner). In a case where the temperature does not change so as not to change the developing efficiency, the amount of charged toner (triboelectricity) can be estimated based on the density of the patch image. Accordingly, by controlling the supply of the toner so as to match the target density of the patch image, the amount of charged toner can be controlled to be the target value.
However, as illustrated in FIG. 6A, when the toner temperature rises, the developing efficiency decreases, and accordingly, when the same control process is performed also in a case where the developing efficiency changes due to a change in the toner temperature, the amount of the charged toner deviates from the target value. More specifically, in a case where the developing efficiency decreases, the density of the image decreases at the same specific charge of toner, and the specific charge of toner is determined to be high. Accordingly, the supply of toner is controlled so as to lower the specific charge of toner.
Therefore, even in a case where the amount of development of the toner (this is determined based on the density of the patch image) is optimized for an image bearing member, the amount of charged toner deviates from the target value, and there is a problem in that the process thereafter cannot be appropriately performed. For example, in order to perform a transfer appropriately, the amount of charged toner is insufficient or excessive, and there is a problem in that the density and the quality of the image are degraded.
Thus, the present invention aims for providing a developing device that can suppress variations in the amount of charged toner due to a change in the developing efficiency that is accompanied with the change in the temperature.